The present invention relates to a printer which forms a latent image having a density graduation in a unit of pixel, then develops the latent image with fine particle toner in liquid developer used for the development, and transfers the developed image onto regular paper so as to obtain a high resolution image thereon.
There have been conventional electrophotography systems such as laser printers, having the following first to fourth configurations:
a first configuration: steps of forming and developing a latent image on a photosensitive medium and transferring a developed image onto a final recording medium for each color are repeated by a number of times equal to a number of different colors to be used, that is, a plurality of color images are superposed with each other on the final recording medium,
a second configuration: images for different colors are formed on respective photosensitive mediums which are prepared each exclusively for the respective different colors, and thereafter, the images on the respective photosensitive medium are superposed with each other on a final recording medium;
a third configuration: latent images are formed on a photosensitive medium for different colors, and are then transferred, for each of colors to be used, onto a secondary image bearing medium such as a dielectric drum or a dielectric belt, so as to successively superposes these images for all colors including a final color on the secondary image bearing medium so as to form a full color image, and the thus formed full color image is transferred onto a final recording medium; and
a fourth configuration: as disclosed in Japanese Laid-Open Patent No. H8-179600 without using a secondary image bearing medium, a latent image which is formed by an ion writing head is developed with toner that is then cured by an ink curing means using ultraviolet radiation or heat, and after images for different colors are superposed with one another on the primary image bearing medium, the images are then transferred onto a final recording medium in a batch.
However, in the conventional technology, for example, as to a color printer having the first configuration, there has been raised such a problem that the printing speed is low. A color printer having the second or third configuration can solve the above-mentioned problems, but has a complicated large scale structure or an increased number of components.
A color printer having the fourth configuration can solve problems of low printing speed, complicated large scale structure, an increased number of components and the like. However, since the color printer having the fourth structure requires ultraviolet radiation or heat which has to be used for curing toner for respective different colors, there is raised a problem of a large power used during development, which has not yet been solved. In the case of curing toner with the use of heat, it has to cope with a defect in the device which would be caused by a temperature rise due to the heat, and with exhaust heat and the like.
The present invention has been devised in view of the above-mentioned problems, and accordingly, an object of the present invention is to provide a color image forming device which can be small-sized without lowering the printing speed and without increasing the power to be used for development, while can minimize waste heat, and as well to provide a color image forming method therefor.
To the end, according to a first aspect of the present invention, there is provided a color image forming device comprising a latent image recording medium on which a latent image is formed, a plurality of developing units for developing the latent image with the use of liquid developers each having at least toner particles and a solvent, the image developed by the developing units being formed on the recording medium, characterized by a drying means for drying the developed image and an adhesion enhancing means for increasing the adhesion of the toner particles.
The adhesion enhancing means desirably applies an electric field which can exert a force directing toward the latent image recording medium to the toner particles constituting an image after the solvent is evaporated, or a force pressing the toner particles against the latent image recording medium. The adhesion enhancing means may be a means for applying a transcription bias. The means for applying a transfer bias may also serve as a transfer bias supply means in a fixing unit, for transferring and fixing a developed image onto a recording medium.
Further, the above-mentioned image forming device preferably incorporates a charge eliminator for eliminating electric charge from an electrostatic recording medium, in order to eliminate an electric charge to a degree by which the gradation of a latent image is not deteriorated.
Further, according to a second aspect of the present invention, there is provided a color image forming device comprising an electrostatic latent image recording medium for carrying thereon electrostatic images corresponding to image signals, a plurality of developing units for manifesting the latent images for different colors with the use of liquid developers, a drier for drying the images for different colors, developed on the electrostatic latent image recording medium by the developers, and an adhesion enhancing means for enhancing adhesion of toner particles on the electrostatic latent recording medium, dried by the dryer.
The adhesion enhancing means includes an adhesion applying means which may be an electric field generating means for generating an electric field for pressing toner particles constituting color images and dried by the drying means, toward the latent image recording medium. This electric field generating means is preferably a transferring means for applying a transfer bias for transfer. An electrostatic latent image for each color formed on the electrostatic latent image is developed by a developing unit corresponding to the color with the use of a liquid developer. The thus developed image wetted with a solvent is dried by the dryer. The dried image is exposed to the electric field which urges toner particles forming the image toward the electrostatic latent image recording medium, and accordingly, the image can be formed in a stable condition on the electrostatic latent image recording medium. Even though a next latent image is formed and is then developed with a second color on the electrostatic latent image recording medium on which the stable image has been formed, the next image can be stably formed without increasing the power used for the device.
Further, the color image forming device according to the present invention, preferably incorporates a transfer means for transferring, in a batch, a final image which is formed on the electrostatic latent image recording medium and which is developed by superposing different color images with one another, onto a final recording medium. This transfer means preferably includes at least one heating means or one pressuring means for fixing the image simultaneously with the transfer.
Further, the color image forming device according to the present invention may have an electric charge eliminating means which preferably eliminates electric charge within a range in which local chare potential difference on the electrostatic latent image recording medium does not deteriorate the gradation of a latent image formed on the latent image recording medium by the recording head.
Further, the color image forming device according to the present invention preferably uses a liquid developer in which the concentration of toner dispersed therein is from 5 to 35%.
According to the present invention, there is provided a color image forming method characterized by the steps of forming an electrostatic latent image corresponding to an image signal, on an electrostatic latent image recording medium, developing the latent image formed on the electrostatic latent image recording medium with the use of a developer reserved in a developing unit, drying the thus manifested image on the developing latent image recording medium, and enhancing the adhesion of toner particles which constitute the dried image.
In the color imager forming method according to the present invention, the step of enhancing the adhesion of the toner particles preferably conveys the electrostatic latent image recording medium formed thereon the electrostatic latent image into an electric field in which the toner particles forming the dried image is pressed toward the electrostatic latent image recording medium.
According to the present invention, images for different colors which have been obtained by developing latent images can be stably laminated with one another with no color mixing. In the case of superposing developers, that is, images having different colors of toner, with one another, when an image having a second color is formed directly over a dried toner image having a first color, there is caused a problem which should be solved. Detailed description will be made of formation of a second latent image on an electrostatic latent image recording medium on which toner having a first color is merely dried in the form of an image, with the use of a recording head. In this condition, the dried toner on the electrostatic latent image recording medium is attracted toward the recording head by an electric field which is applied between the electrostatic latent image recording medium and the recording head in order to form a new latent image, resulting in a damage to the recording head. In order to prevent occurrence of such a problem, according to the present invention, before the second image is formed over the image formed by the dried toner, the toner is pressed toward the electrostatic latent image recording medium by the electric field. Due to the above-mentioned pressing, the physical adhesion between the toner and the electrostatic latent image recording medium and in the toner can be enhanced, and accordingly, there can be prevented the scattering of toner from the recording head by an electric field, color mixing in the developing part, and the like, thereby it is possible to carry out superposition of the toner images on the electrostatic latent image recording medium only through drying of the liquid developer without curing of toner (melting or solidifying of toner particles).
Further, in the color image forming method according to the present invention, the above-mentioned steps are repeated by several times for different colors, except a final color in the developing unit, and an image for this final color which is developed on the electrostatic latent image recording medium, and is then dried, may be transferred onto a final recording medium.
Further, in the color image forming method according to the present invention, the above-mentioned steps are repeated by several times for different colors, except a final color in the developing unit, and an image for this final color which is developed on the electrostatic latent image recording medium, may be transferred onto a final recording medium without being dried.